1. Field of the Invention
This invention generally relates to a coupling device for connecting shafts together for power transmission, and particularly to a universal joint for transmitting rotating power between two shafts. More specifically, the present invention relates to a universal joint particularly suited for use in a rotating mechanism of wheel printer for rotating a print wheel to have a selected type located at the printing position.
2. Description of the Prior Art
A universal joint as a linkage for transmitting rotating power between two shafts is well known in the art. A universal joint is often used in a wheel printer in which a print wheel comprising a hub, a plurality of spokes extending radially outwardly from the hub and a plurality of type elements provided at the forward ends of the spokes is rotatably supported. That is, in a wheel printer, a universal joint is used to operatively couple a driving shaft, which is driven to rotate, for example, by means of a motor, to a driven shaft on which is fixedly mounted the print wheel. In such a universal joint utilized in wheel printers, it is often so structured that the one of the two shafts coupled by the universal joint may shift in parallel with respect to the other. Such a structure is particularly advantageous if use is made of a double circular print wheel which has type elements arranged in two circles, inner and outer, because the print wheel must be moved in the direction perpendicular to its rotating axis so as to locate a selected type element in either inner or outer circle at a predetermined printing position.
In the case of the universal joint having the above-described structure, it is important that the shafts may shift in parallel with respect to each other quite smoothly. If not, imprints will be poor in quality and an increased amount of power will be required. In addition, the speed of printing operation and its service life will be limited.